c++对象虚表

虚表(virtual table)

The virtual table is actually quite simple, though it’s a little complex to describe in words. First, every class that uses virtual functions (or is derived from a class that uses virtual functions) is given it’s own virtual table. This table is simply a static array that the compiler sets up at compile time. A virtual table contains one entry for each virtual function that can be called by objects of the class. Each entry in this table is simply a function pointer that points to the most-derived function accessible by that class.

带有虚函数的类都有自己的虚表，虚表是编译器在编译时就确定的一个静态数组。虚表中包含每个虚函数的一个入口(函数指针)，且这个函数指针是对象能访问到最近的一个函数指针。

Second, the compiler also adds a hidden pointer to the base class, which we will call *__vptr. *__vptr is set (automatically) when a class instance is created so that it points to the virtual table for that class. Unlike the *this pointer, which is actually a function parameter used by the compiler to resolve self-references, *__vptr is a real pointer. Consequently, it makes each class object allocated bigger by the size of one pointer. It also means that *__vptr is inherited by derived classes, which is important.

编译器会为基类设置一个隐藏的指针(*__vptr), 该指针是在类实例化时自动设置指向类的虚表的。

1、在进入构造函数时，虚表已经构建完成。

2、虚构时，虚表可以回滚。

3、虚表与虚表指针的区别：每个类只有一个虚表，每个对象实例都有个虚表指针。

如下：

#include "stdafx.h"
#include <iostream>

class CBase
{
public:
    CBase()
    {
        std::cout<<"CBase::CBase()"<<std::endl;
        print();
        //print1();
    }
    virtual ~CBase()
    {
        std::cout<<"CBase::~CBase()"<<std::endl;
        print();
    }
    //virtual void print1() = 0;
    virtual void print()
    {
        std::cout<<"CBase::print()"<<std::endl;
        //print1();
    }
};

class CDerived : public CBase
{
public:
    CDerived()
    {
        std::cout<<"CDerived::CDerived()"<<std::endl;
        print();
    }
    
    virtual ~CDerived()
    {
        std::cout<<"CDerived::~CDerived()"<<std::endl;
        print();
    }

    virtual void print()
    {
        std::cout<<"CDerived::print()"<<std::endl;
    }

    virtual void print1()
    {
        std::cout<<"CDerived::print1()"<<std::endl;
    }
};

void DoPrint(CBase* pBase)
{
    pBase->print();
}
int _tmain(int argc, _TCHAR* argv[])
{
    CBase* pBase = new CBase;
    CDerived* pDerived = new CDerived;
    CBase* pBase1 = pDerived;

    std::cout<<"---------"<<std::endl;
    DoPrint(pBase);
    DoPrint(pDerived);
    DoPrint(pBase1);
    std::cout<<"---------"<<std::endl;

    delete pBase;
    delete pDerived;

    return 0;
}

　　输出：

CBase::CBase()

CBase::print()

CBase::CBase()

CBase::print()

CDerived::CDerived()

CDerived::print()

---------

CBase::print()

CDerived::print()

CDerived::print()

---------

CBase::~CBase()

CBase::print()

CDerived::~CDerived()

CDerived::print()

CBase::~CBase()

CBase::print()